There is no change in internal energy of an ideal gas when it undergoes

Which of the following processes is reversible?

For the free expansion of a gas,which of the following is true?

Two cylinders $A$ and $B$ of equal capacity are connected to each other via a stopcock. $A$ contains a gas at standard temperature and pressure. $B$ is completely evacuated. The entire system is thermally insulated. The stopcock is suddenly opened. Answer the following :
$(a)$ What is the final pressure of the gas in $A$ and $B$?
$(b)$ What is the change in internal energy of the gas?
$(c)$ What is the change in the temperature of the gas?
$(d)$ Do the intermediate states of the system (before settling to the final equilibrium state) lie on its $P-V-T$ surface?

Which of the following is not reversible?

$Assertion :$ Thermodynamic processes in nature are irreversible.
$Reason :$ Dissipative effects cannot be eliminated.